Fix stdlib stubtest flakes

[AlexWaygood]

See #14870 (comment). find_distributions seems to sometimes be available in CI, and sometimes not. Use the "trick" of a regular expression that makes the allowlist entry option (as described in https://github.com/python/mypy/pull/20007/files, which will be published as part of the official stubtest docs after mypy cuts its next release).

Meanwhile, the GitHub build of Python 3.13.9 (just released) on macOS doesn't appear to have errno.ENOTCAPABLE on it. We already ignore this stubtest error on macOS on other Python versions; let's just do the same here:

typeshed/stdlib/@tests/stubtest_allowlists/darwin-py311.txt

Lines 42 to 47 in d119712

	# ================
	# Unclear problems
	# ================
	# Added in 3.11.1, flagged by stubtest on Python < 3.14 for unknown reasons
	errno.ENOTCAPABLE

typeshed/stdlib/@tests/stubtest_allowlists/darwin-py312.txt

Lines 30 to 35 in d119712

	# ================
	# Unclear problems
	# ================
	# Added in 3.11.1, flagged by stubtest on Python < 3.14 for unknown reasons
	errno.ENOTCAPABLE

[srittau]

$#&%R^W# stubtest

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉